This invention relates to a residual current detection device for use in a circuit breaker.
Conventionally, residual current is detected utilizing a current transformer having primary windings through which, in the case of a single phase device, load current flows in opposite directions so that if the return current is different from the outwardly flowing current because of current leakage an output current signal is induced in a secondary winding of the transformer. In the case of a multi-phase device, primary windings of the transformer are connected in all of the phase lines and the neutral line. In normal situations, when there is no current leakage, the net current induced in the secondary winding is zero and therefore no output is detected.
Sophisticated materials have been developed for the core of the current transformer, which enable considerable accuracy to be obtained when the currents flowing in the primary windings are substantially sinusoidal. However, switch mode power supplies are often used for computers and other equipment and there is an increasing tendency for such equipment to cause dc offsets in the currents. Such developments have made detectors utilizing current transformers less reliable and prone to false tripping or failure to detect a dc current leakage.
This is a particular problem in the case of directly actuated electro-mechanical devices, where the current transformer secondary winding actually drives an actuator. The situation is not much improved, when including an electronic detection and amplification means connected to the secondary winding, as there are still problems with high frequency transients and de offsets. A very small dc current level can cause the core to saturate thereby seriously impairing the ability of the detector to detect current leakage.
It is an object of the present invention to provide a residual current detection device in which the above mentioned problems are substantially overcome in a simple and efficacious manner.
In accordance with the invention there is provided a residual current detection device comprising a plurality of resistive shunts for connection in respective ones of a plurality of lines though which current can flow to and from a load, and detector means sensitive to the voltage developed across each of the shunts to detect any imbalance between the currents flowing though the shunts.
Preferably, the detector means comprises an analog to digital converter for each shunt and a processor for receiving the digital signals from the converters and determining whether a current imbalance exists.
Each shunt preferably takes the form of a composite strip having conductive portions at its ends and a resistive portion interconnecting the conductive portions. Such composite strips can be mass produced inexpensively to very high tolerances which makes them extremely suitable for this purpose.
The analog to digital converter for each shunt may include a delta-sigma modulator, which generates a high frequency single digital data stream which is converted by decimation filtering to a multibit digital data stream at a lower frequency.
The analog to digital converter for each shunt is preferably connected to the processor through an isolation barrier so that the converter can float at the voltage level of the shunt which it serves. The decimation filtering may be effected entirely in the converter, entirely in the processor or split between the converter and the processor.